High impact polystyrene (HIPS) was tested with different combinations of two PIN flame retardants, inorganic magnesium hydrate (MH) and microencapsulated red phosphorus (MRP). Fire performance was assessed using LOI (limiting oxygen index), UL-94 (3mm thickness) and cone calorimeter (heat release rates). Highest (best) LOI and UL-94 V0 were achieved at c. 23% MH plus 10% RDP. It is noted however, that smoke and carbon monoxide release increase with MRP loadings.
The authors consider that this is because this PIN FR acts principally in the gas phase by release of gaseous phosphorus compounds which interfere with combustion gases.
“Influence of microencapsulated red phosphorus on the flame retardancy of high impact polystyrene/magnesium hydroxide composite and its mode of action”, J. Liu et al., Polymer Degradation and Stability 121 (2015) pages 208-221, http://dx.doi.org/10.1016/j.polymdegradstab.2015.09.011